1. Field of the Invention
The present invention relates to a nitride semiconductor device, and more particularly, to a nitride semiconductor device having an active layer of a superlattice structure.
2. Description of the Related Art
In general, a nitride semiconductor is widely used in a green or blue light emitting diode (LED) or a laser diode (LD) serving as a light source for full-color displays, image scanners, various signal systems and optical communication devices. This nitride semiconductor device generates and emits light in an active layer by recombination of electrons and holes.
The active layer features a single quantum well (SQW) structure having a single quantum well layer and a multi quantum well (MQW) structure having a plurality of quantum well layers each having a thickness smaller than 100□. Notably, the active layer of the MQW structure exhibits higher optical efficiency with respect to current and better light emitting output than the active layer of the SQW structure. This has brought about active utilization of the MQW structure.
Optical efficiency of this nitride semiconductor device is basically determined by internal quantum efficiency, i.e., a recombination rate of electrons and holes in the active layer. Studies for achieving higher internal quantum efficiency have been focused chiefly on improving a structure of the active layer itself or increasing effective mass of carriers.
However, the electrons and holes, if non-uniform with respect to each other in mobility, cause some carriers not to be recombined in the active layer, thereby lowering a recombination rate for desired light emission inside the active layer.
Therefore, to increase effective mass of the carriers in the active layer, a smaller number of carriers should be recombined outside the active layer. Especially, to maximize a capture rate of electrons and holes, there should be a method in place to confine the electrons which are relatively more mobile than the holes, inside the active layer.